The concept of inheritance of traits is widely studied in genetics. It is he who explains the similarity of offspring and parents. It is curious that some manifestations of signs are inherited jointly. This phenomenon, first described in detail by the scientist T. Morgan, became known as “linked inheritance”. Let's talk about it in more detail.
As you know, each organism has a certain number of genes. At the same time, a chromosome is also a strictly limited number. For comparison: a healthy human body has 46 chromosomes. Genes in it are thousands of times more. Judge for yourself: each gene is responsible for one or another trait that manifests itself in the appearance of a person. Naturally, there are a lot of them. Therefore, they began to talk about the fact that several genes are localized on the same chromosome. These genes are called a linkage group and determine linked inheritance. A similar theory has been in the scientific community for quite some time, but only T. Morgan gave it a definition.
In contrast to the inheritance of genes that are localized in different pairs of identical chromosomes, linked inheritance causes the formation of a diheterozygous individual of only two types of gametes repeating the combination of parental genes.
Along with this, gametes arise, in which the combination of genes differs from the chromosome set of parents. This result is a consequence of crossing over, a process whose importance in genetics is difficult to overestimate, since it allows offspring to receive various traits from both parents.
In nature, there are three types of gene inheritance. In order to determine which type is inherent in this particular pair, analyzing crosses are used . As a result, you will definitely get one of the three options below:
1. Independent inheritance. In this case, hybrids differ from each other and from parents in appearance, in other words, as a result, we have 4 phenotypes.
2. Complete linkage of genes. Hybrids of the first generation, obtained by crossing parental individuals, completely repeat the phenotype of parents and are indistinguishable among themselves.
3. Incomplete linkage of genes. As in the first case, when crossing, 4 classes of different phenotypes are obtained. At the same time, however, the formation of new genotypes that are completely different from the parent fund. It is in this case that the crossingover mentioned above intervenes in the formation of gametes.
It was also found that the smaller the distance between the inherited genes in the parent chromosome, the higher the likelihood of their complete linked inheritance. Accordingly, the farther they are from each other, the less often a cross occurs during meiosis. Gen spacing is a factor that primarily determines the likelihood of linked inheritance.
Separately, gender-linked inheritance should be considered. Its essence is the same as in the case considered above, however, the inherited genes in this case are located on the sex chromosomes. Therefore, talking about this type of inheritance is possible only in the case of mammals (humans among them), some reptiles and insects.
Taking into account the fact that XY is a set of chromosomes corresponding to the male sex, and XX to the female, we note that all the main characters responsible for the vitality of an organism are located on the chromosome present in the genotype of each organism. Of course, we are talking about the X - chromosome. In female individuals, both recessive and dominant genes in the chromosomes can be present. Men can inherit only one of the options - that is, either the gene manifests itself in the phenotype or not.
Coupled inheritance due to sex often sounds in the context of diseases that are specific to men, while women are only their carriers:
- hemophilia,
- color blindness;
- Lesch-Nyhan syndrome.